In recent years, various types of fuel cell apparatuses, in which a fuel cell stack including a plurality of fuel cells that are electrically connected in series is housed in a container, are being proposed as a next-generation energy.
FIG. 3 shows a conventional solid electrolyte fuel cell stack, which fuel cell stack has a configuration in which a plurality of fuel cells 21 (21a, 21b) are aligned and assembled, and a current collector member 25 made of a metal felt is interposed between a fuel cell 21a and another fuel cell 21b so that a fuel-side electrode 27 of the fuel cell 21a and an oxygen-side electrode 23 of the other fuel cell 21b are electrically connected.
Further, the fuel cell 21 (21a, 21b) has a configuration in which a solid electrolyte 29 and an oxygen-side electrode 23 made of conductive ceramics are disposed sequentially on the outer periphery of a cylindrical fuel-side electrode 27 made of a metal. An interconnector 22 is disposed on the fuel-side electrode 27 that is exposed through the solid electrolyte 29 and the oxygen-side electrode 23 so as to prevent a connection with the oxygen-side electrode 23. The interconnector 22 is electrically connected to the fuel-side electrode 27.
This interconnector 22 is formed of conductive ceramics that is dense and is difficult to transform by a fuel gas and an oxygen-containing gas, in order to reliably insulate the fuel gas that flows through the fuel-side electrode 27 and the oxygen-containing gas that flows outside the oxygen-side electrode 23 from each other.
The electrical connection between the fuel cell 21a and the other fuel cell 21b is established by connecting the fuel-side electrode 27 of the fuel cell 21a to the oxygen-side electrode 23 of the other fuel cell 21b via the interconnector 22 disposed on the fuel-side electrode 27, and the current collector member 25.
Furthermore, a fuel cell apparatus is configured by housing the above-described fuel cell stack into a container, and generates power at about 1000° C. by feeding fuel (hydrogen) into the fuel-side electrode 27 and air (oxygen) into the oxygen-side electrode 23.
In such a fuel cell 21, generally, the fuel-side electrode 27 is made of ZrO2 (YSZ) that contains N1 and Y2O3, the solid electrolyte 29 is made of ZrO2 (YSZ) that contains Y2O3, and the oxygen-side electrode 23 is formed of a LaMnO3-based composite oxide in which Sr (strontium) coexists.
Also, a production method has been proposed recently in which a solid electrolyte and an oxygen-side electrode are co-sintered (co-fired). However, the co-sintering of a solid electrolyte and an oxygen-side electrode is problematic in that a component (e.g., Sr, or the like) contained in the oxygen-side electrode diffuses into the solid electrolyte, forming a reaction layer having a high electrical resistance on the interface between the solid electrolyte and the oxygen-side electrode, and as a result, causing performance degradation of the fuel cell.
In view of this, for the purpose of preventing performance degradation of fuel cells caused by co-sintering of the solid electrolyte and the oxygen-side electrode, a fuel cell in which an intermediate layer is formed between the solid electrolyte and the oxygen-side electrode, and a method for producing such a fuel cell have been proposed (see, for example, Japanese Unexamined Patent Publications JP-A 2003-288914 and JP-A 2004-63226).
Also, in order to provide a solid electrolyte fuel cell apparatus that has superior durability against heat cycles and provides sufficient power generation performance, a solid electrolyte fuel cell apparatus has been proposed in which a solid electrolyte layer, an reaction-preventing layer, a mixing layer, and an air electrode layer are laminated sequentially on a surface of a fuel electrode substrate, and the mixing layer contains the materials of the reaction-preventing layer and the air electrode layer (see, for example, Japanese Unexamined Patent Publication JP-A 2005-327637).
Furthermore, in order to provide a fuel cell apparatus that has superior durability and power generation performance, a solid electrolyte fuel cell apparatus has been proposed in which a solid electrolyte, an reaction-preventing layer, and an air electrode layer are laminated sequentially on the upper surface of a fuel electrode substrate, the reaction-preventing layer includes a first reaction-preventing layer and a second reaction-preventing layer having pores, and the solid electrolyte layer, the first reaction-preventing layer and the second reaction-preventing layer are co-sintered (see, for example, Japanese Unexamined Patent Publication JP-A 2005-327507).
However, even when an intermediate layer made of a single layer is formed between the solid electrolyte and the oxygen-side electrode, it is still problematic in that a situation can occur in which a component (Zr, zirconium) contained in the solid electrolyte diffuses into the intermediate layer and a component (Sr) contained in the oxygen-side electrode diffuses into the solid electrolyte, and when power generation is continued for a long period of time, the diffused solid electrolyte component reacts with the component that has diffused from the oxygen-side electrode, forming a reaction layer having a high electrical resistance, which causes power generation performance degradation of the fuel cell apparatus. In addition, there is another problem in that the component contained in the oxygen-side electrode diffuses into the solid electrolyte, and the oxygen-side electrode component that has been thus contained into the solid electrolyte reacts with the solid electrolyte component, forming a reaction layer having a high electrical resistance, and as a result, causing power generation performance degradation of the fuel cell apparatus.
Furthermore, in the case of a fuel cell in which the solid electrolyte is fired first, and after that, the intermediate layer is fired, there is a problem in that sufficient fixation between the solid electrolyte and the intermediate layer is not obtained, and when the fuel cell apparatus is operated for a long period of time for power generation, a separation occurs between the solid electrolyte and the intermediate layer, and as a result, causing power generation performance degradation of the fuel cell apparatus.
Furthermore, even when an reaction-preventing layer is disposed on the surface of the solid electrolyte layer and a mixing layer containing an oxygen-side electrode component is disposed on the surface of the reaction-preventing layer in order to prevent the separation between the solid electrolyte layer and the oxygen-side electrode as disclosed in JP-A 2005-327637, when the fuel cell apparatus is operated for a long period of time for power generation, a problem arises in that an oxygen-side electrode component or oxygen-side electrode component contained in the mixing layer reacts with a solid electrolyte component that has diffused into the reaction layer, or the oxygen-side electrode component diffuses into the solid electrolyte and reacts with the solid electrolyte component, forming a reaction layer having a high electrical resistance, and as a result, causing power generation performance degradation of the fuel cell apparatus.
Also, even when two reaction-preventing layers are formed as disclosed in JP-A 2005-327507, a solid electrolyte component (Zr) can diffuse into the reaction-preventing layer (second reaction-preventing layer) during the process of co-sintering a solid electrolyte, a first reaction-preventing layer and a second reaction-preventing layer, and when power generation is continued for a long period of time, the solid electrolyte component (Zr) that has diffused into the reaction-preventing layer (the second reaction-preventing layer) reacts with the oxygen-side electrode component (Sr), forming a reaction layer having a high electrical resistance, and as a result, causing power generation performance degradation of the fuel cell apparatus.